1. Field of Invention
This invention relates generally to the use of species removal media for efficient removal of large quantities of an unwanted species from a process stream containing the unwanted species, and the subsequent regeneration of at least a portion of the species removal media for its reuse for further unwanted species removal. The species removal media can be a precipitating chemical solvent and can be used for efficient removal of large quantities of CO2 from a process stream, and the subsequent regeneration of the chemical solvent with precipitates so that it could be reused for further CO2 removal. Specifically, the present invention is a unique regenerator and a novel process for efficient handling and regeneration of CO2-rich chemical solvents that contain precipitated reaction products from absorber.
2. Background and Related Art
Many chemical production processes utilize regenerable chemical solvents to remove acid gases from product streams or other process streams. For example, the essential components of a process for acid gas removal includes two reactors: an absorber and a regenerator. The acid gas in the process stream is absorbed into the chemical solvent in the absorber and is desorbed from the solvent in the regenerator. The regenerated solvent is recycled back to the absorber for reuse, and therefore, the solvent is called a regenerable chemical solvent.
Such use of regenerable chemical solvents is being adopted for capture and sequestration of carbon dioxide from power plant flue gases. Also, CO2 capture is being realized using regenerable chemical solvents in precombustion processes such as the integrated gasification combined cycle (IGCC) process for power generation. Typically, such traditional chemical solvents are regenerated for reuse utilizing steam as a stripping agent and as a source of heat for the endothermic chemical reactions. Yet, while effective, such regeneration systems for CO2 capture are energy intensive.
Many different species removal media exist to remove acid gases from process streams, for example, chemical solvent processes disclosed in “Gas Purification” by Kohl and Nielsen (Gulf Publishing, 1997). Among them, acid gases are most typically removed using aqueous amines. The methyl diethanol amine (MDEA) in various formulations is the most widely used chemical solvent in the natural gas, refinery gas and synthesis gas industries. A reformulated amine based solvent absorbs CO2 via various chemical reactions producing a rich chemical solvent that is then regenerated at a higher temperature. As used herein, the relative terms ‘lean’ and ‘rich’ reflect the state of the species removal media with, for example, low and high concentrations of CO2.
Unlike typical acid gases such as H2S targeted for removal in the IGCC process, CO2 is present in much larger quantities. The H2S in the syngas is typically in ppm levels at the inlet of the acid gas removal unit, whereas CO2 concentrations in fully shifted syngas is typically up to 25 mole percent from air blown gasification, and up to 40 mole percent from oxygen blown gasification. The flue gas from a pulverized coal (PC) combustion plant also contains large quantities of CO2, typically in the approximately 12 to 15% range. Further, the volume of the flue gas from a power plant is large—an 880 MWe coal-burning plant can generate flue gas at a rate of more than 120 million ft3/hr.
As the absorption capacity of any given solvent is essentially constant, and large quantities of CO2 needs to be separated from the syngas or flue gas, the quantity of chemical solvent that needs to be circulated between the absorber and regenerator also is proportionally high. With conventional technology, such high solvent flow rates between the absorber and regenerator would require large amounts of regeneration energy to break the chemical bond between the CO2 and solvent molecules to regenerate the solvent. In addition, water used to form the solvent would be heated up in the regeneration process, requiring additional energy consumption. Thus, the conventional acid gas removal processes using conventional solvents become uneconomical for CO2 removal from power plant processes due to vast regeneration energy requirements.
The absorption and regeneration apparatus for conventional reformulated chemical solvent processes for acid gas removal comprises tray or packed towers. These towers, especially towers with structured packing, are widely used in the process gas industries as their operation and performance are well characterized. However, the tray and packed towers are prone to plugging if precipitates or solids are present in the solvent. The process is designed and operated at conditions to limit if not avoid forming precipitates during absorption process. In case precipitate formation, the conventional process can include solvent filters to filter out solids that may be present in the solvent circulating around the loop between the absorber tower and the regenerator tower. Yet, the prevalent use of such an apparatus limits the use of solvents to non-precipitating chemical solvents.
The effectiveness of a particular species removal media comprising an aqueous chemical solvent to minimize energy consumption depends upon the concentration of the active absorbing component in the solvent. For example, due to the highly corrosive nature of monoethanol amine (MEA), MEA systems are typically operated at a low concentration of about 15 to 30 wt % MEA in water compared to reformulated MDEA solvent that can be up to 50 wt %. As the solvents need to be heated for regeneration, MEA solvent requires more energy to heat up the large quantity of water in the solvent. Additionally, the low concentration of amine in MEA system leads to higher solvent circulation rates between the absorber and regenerator, leading to more regeneration energy consumption.
The effectiveness of an aqueous chemical solvent is also determined by the extent to which the active absorbing component is utilized. Full utilization of the active component will lead to lower solvent circulation rates between the absorber and the regenerator, and consequently lower regeneration energy consumption.
Absorption of CO2 using aqueous ammonia solvent in pre-combustion IGCC or PC combustion process leads to the formation of ammonium carbonate (one mole of CO2 absorbed per mole of ammonia). Complete utilization of solvent is realized with further absorption of CO2 leading to the formation of ammonium bicarbonate (two moles of CO2 absorbed per mole of ammonia). With its attendant limited solubility, ammonium bicarbonate precipitates out. As used herein, the terms precipitated ‘salts’ and ‘crystals’ are used interchangeably, with both the terms referring to suspended solids in the solvent solution. If the absorber and regenerator are capable of handling precipitated solutions, lower regeneration energy consumption can be realized when the solvent is fully utilized. U.S. Patent Publication No. 2012/0216680, which is herein incorporated by reference, discloses a circulating dispersed bubble absorber that is capable of handling precipitating solvents upon absorbing CO2.
As with aqueous ammonia solvent, several amino acid salt solutions (solvents) produce precipitates upon absorbing CO2, (Feron and ten Asbroek, Green House Gas Technology Conference, 2004. These chemical solvents are stable, highly reactive and require lower regeneration energies as precipitation leads to high CO2 loadings. The challenge is to configure a suitable absorber and regenerator to handle precipitating solvents such as aqueous ammonia and amino acid salt solutions.
U.S. Patent Publication No. 2009/0081096 discloses the absorption of CO2 from process gas in hydrated lime solvent to form insoluble calcium carbonate precipitates. The precipitates are separated from the solution and are sold or sequestered without regeneration. Due to practical difficulties, instead of regenerating the solvent from the precipitates, mined limestone is continuously calcined to form the hydrated lime solvent. Yes, such a one-use process of the hydrated lime solvent increases cost significantly without the full benefit of overall CO2 capture, as the calcination process to produce solvent generates additional CO2.
To overcome the operability, efficiency and cost issues mentioned above, an improved species removal media for the efficient removal of large quantities of an unwanted species from a process stream containing the unwanted species, and the subsequent regeneration of at least a portion of the species removal media for its reuse for further unwanted species removal is highly desirable. Further still, a process that can regenerate chemical solvents that contain precipitated salts that formed upon absorption of CO2 from an IGCC or PC combustion process stream is highly desirable. It is the intention of the present invention to provide for such industrial needs.